One thing struck me as I played a normal match this morning before work (or tried to) - How your success rate goes dosen't depend on difficulty level. It's 80% teamwork. On No Mercy, I had a guy this morning sit outside the safe room on the subway level and pick off boss infected for 10 minutes at 1% health before continuing the campaign, even with 3 other people telling him to get a move on.
On the other hand, same team ran into a tank in the Sewers and was cremated within 10 seconds, leaving me to stumble around trying to pump enough lead into it to save the game... and I failed, as it's tough on any difficulty to solo a tank in the Sewers. Too narrow and not well enough lit to run.
I'm only playing No Mercy until I can succeed at it. Overall, this game is going to keep me occupied for a very long time. Feel free to friends list me on Steam if you want a game. I promise not to friendly fire you!